It is said that today is the age of information, and high-density and large-capacity memories at the heart thereof have been developed enthusiastically. Memories are required to have not only the capability of high-density and large-capacity storage, but also the capability of achieving high reliability, rewriting capability and the like. Among recording media that satisfy these requirements, optical disk memories such as a magneto-optical disk and the like have received the most attention. The present invention relates to an optical disk recording/reproducing device that performs recording or reproduction with respect to such optical disk memories.
Conventionally, many technical reports have been made on optical disk recording/reproducing devices. The following description will be made with reference to the appended drawings by using a Mini Disc device as an example of optical disk recording/reproducing devices with the rewriting capability among various types of optical disk recording/reproducing devices.
FIG. 14A is a plan view schematically showing the appearance of an optical head and a magnetic head of an optical disk recording/reproducing device that performs recording/reproduction with respect to a recording medium that is a magneto-optical disk such as a Mini Disc or the like. FIG. 14B is a side view showing the appearance. The following description is directed to the configuration and operation with reference to these figures.
In FIGS. 14A and 14B, reference character 1 denotes a light receiving/emitting element that is configured as a single device mounting a semiconductor laser chip and an optical signal detecting part therein. The semiconductor laser chip is a light emitting part that emits laser light, and the optical signal detecting part receives reflected light from a recording medium 8 that originates in this laser light so as to detect various signals. Reference character 2 denotes a laser beam that is radiated from the light receiving/emitting element 1. Reference characters 3 and 4 denote a mirror that leads a laser beam from the light receiving/emitting element 1 to the recording medium 8, and an objective lens actuator that shifts an objective lens 5 in a tracking direction and a focusing direction so that the objective lens 5 follows the eccentricity and surface wobbling of the recording medium, respectively. Reference characters 4a, 4b, 4c and 4d denote a magnet that constitutes a movable part of the objective lens actuator 4, a coil for allowing a driving force to be generated in the magnet 4a, a fixed part of the objective lens actuator 4, and an actuator base for fixing the objective lens actuator 4 to an optical base 6, respectively. Further, reference characters 5 and 6 denote the objective lens that focuses the laser beam 2, which has been reflected off the mirror 3, onto the recording medium 8 so that a minute beam spot is formed, and the optical base for fixing the light receiving/emitting element 1 and the objective lens actuator 4, respectively. Reference character 7 denotes a magnetic head that, in the case where the recording medium 8 is of a recording type, applies a modulated magnetic field to the recording medium 8 so as to realize so-called magnetic field modulation recording. The magnetic head 7 is composed of a magnetic core 7a that is formed of a magnetic material, a coil 7b, a sliding part 7c that slides on the recording medium and keeps the magnetic core 7a at a given distance from a surface of the recording medium 8, and a supporting part (not shown). Reference character 8 denotes the recording medium. In FIG. 14A, an arrow X indicates the tracking direction of the recording medium 8 (namely, a radial direction of the disk-like recording medium 8).
In the optical disk recording/reproducing device having the above-mentioned configuration, when performing reproduction, the laser beam 2 is emitted from the light receiving/emitting element 1, and the objective lens actuator 4 is driven so that the objective lens 5 forms a minute beam spot in a predetermined position of the recording medium 8. A reflected light beam from the recording medium 8 returns to the light receiving/emitting element 1, so that a focus error signal, a tracking error signal, and a RF signal are detected. When performing recording, the light receiving/emitting element 1 emits an optical power having a given intensity so that the temperature of an information recording film is raised to a temperature not lower than the Curie point by a beam spot formed by focusing a light beam on the recording medium 8. Further, a modulated current having a waveform shown in FIG. 16 is applied to the magnetic head 7 that is provided on a side opposite to a light beam incidence side with respect to the recording medium 8. This allows the recording film, which has been heated to a temperature not lower than the Curie point, to be magnetized perpendicularly, so that so-called magnetic field modulation recording is performed.
The coil 7b of the magnetic head 7 is supplied with an electric current represented by FIG. 16, and thus magnetic flux is emitted from an end of the magnetic core 7a. A distribution of a magnetic field intensity obtained in this case is shown in FIG. 15. FIG. 15 shows the results obtained by determining a magnetic field intensity in the vicinity of a beam spot. In the figure, the horizontal axis indicates a distance D in a radial direction from a central position of the magnetic core 7a, and the vertical axis indicates a magnetic field intensity expressed as a unitless value relative to the magnetic field intensity required for recording.
In FIG. 15, in an area defined by a distance of ±0.5 mm in the radial direction from the central position of the magnetic core 7a, a constant magnetic field intensity is obtained. A width defined by this area substantially equals a width (1 mm) of the magnetic head 7 in the radial direction. In a region defined by an absolute value of the distance D in the radial direction from the central position of the magnetic core 7a higher than 0.5 mm, the magnetic field intensity decreases with increasing distance. The magnetic field intensity required for recording changes depending on a distance between a beam spot and the magnetic head (a thickness of a protective layer on the recording medium, an assembly error, an orientation of the magnetic head, or the like) and by a shift of the objective lens in the tracking direction.
In the above-mentioned configuration according to the conventional technique, the driving current for the magnetic head 7 is set so as to allow recording to be performed under any assumed condition. That is, the current value of a driving current to be supplied to the coil 7b was set so that a magnetic field intensity that enables recording was obtained by giving consideration to a distance between a beam spot and an end of the magnetic core and a maximum distance the objective lens is shifted in the radial direction from the central position of the magnetic core.
In this example, the recording medium 8 is set so as to have a maximum amount of eccentricity of 0.6 mm. In order to allow a beam spot to follow a recording track of the recording medium 8 having such an amount of eccentricity, the objective lens 5 is shifted a distance in a range of ±0.6 mm in the radial direction with respect to the magnetic core 7. Therefore, it is necessary that a magnetic field intensity required for recording should be obtained in an area defined by a distance of ±0.6 mm in the radial direction from the central position of the magnetic core 7a (“effective magnetic field region” shown in FIG. 15). To this end, an electric current having a higher current value was applied to the coil 7b so that a magnetic field intensity of not lower than 1 was obtained in the area defined by the distance of ±0.6 mm for the distance D in the radial direction as shown by a dotted line in FIG. 15. Accordingly, in an area defined by a distance of ±0.5 mm for the distance D in the radial direction from the central position of the magnetic core, a magnetic field intensity of 1.25 is obtained. This indicates that when a beam spot is positioned within this area, an electric current having a current value higher than necessary is applied. As a result of this, power consumption is increased, and thus in portable equipment, the continuous operation time is shortened, which has been disadvantageous.
Meanwhile, in the case where the magnetic core has a width in the radial direction that is increased so as to correspond to a shift amount of the objective lens 5 in the radial direction (±0.6 mm in the above-mentioned example), the inductance of the coil increases. Recent years have seen a trend in which recording is performed at a higher transfer rate and thus requires a higher recording frequency. Despite such a trend, with the inductance increased, a higher recording frequency cannot be attained, which has been disadvantageous.